An alleged rogue trader accused of Britain's biggest banking fraud fooled his bank's own regulators into believing there was an innocent explanation for serious irregularities in his deals, a court heard.

Kweku Adoboli, 32, who is accused of gambling away £1.4bn while working as a trader for UBS, was questioned after back-office staff spotted billions in incorrectly booked trades weeks before he was arrested. Jurors at Southwark crown court heard Adoboli told compliance staff that he failed to properly book deals to save time because he was short-staffed and "really fucking busy".

He even managed to convince accountants, but was unable to provide figures to back it up, the court heard.

Three weeks later he sent a "bombshell" email to colleagues admitting the trades were "off-book," or "fake". He said he had hoped to make his losses back, but had "clearly failed", adding: "I am deeply sorry to have left this mess for everyone and to have put … my colleagues at risk."

Giving evidence to the trial, UBS compliance staff member William Steward, who was asked to look into the financial irregularities, said that until he received the email on 14 September he had never considered that the trades were off book. He said: "No. That means to me they are not real deals. They don't have a customer, anyone on the other side. They are fake."

He told jurors Adoboli's explanation that he had not booked trades in properly due to staff shortages initially appeared plausible. During the calls on 24 August last year, which were played to the jury, Adoboli explained that he had cut corners in his trades to make up time.

He said: "The main problem going through July and August is that we were really really fucking busy. We had an annoying client which was taking up half my day plus we had all these client meetings and the market was going crazy, plus I was like a man or two down every day."

By the end of the third conversation, Steward told the court, he understood Adoboli's explanations but requested to see figures to back them up. Three weeks later he received an email from Adoboli which revealed his earlier explanations were false.